1. Field of the Invention
The present invention relates to scintillation counting and, more particularly, to apparatus for positioning a standard radioactive source proximate the counting chamber of a liquid scintillation counter.
2. Description of the Prior Art
Liquid scintillation techniques have been widely adopted to measure the count rate or activity of samples containing radionuclides. It is well known that materials present in the sample can affect the process by which the liquid scintillation solution responds to the radionuclide radiation thereby causing a reduction in the measured count rate. This is commonly referred to as quenching, and numerous techniques have been developed in the prior art to correct for quench in order to accurately determine the activity of the sample. One of the more widely adopted techniques is that of external standard channels ratio (ESCR) determination exemplified by U.S. Pat. No. 3,381,130. A second and pioneering technique, commonly termed H-number determination, represents an improvement over the ESCR approach and is exemplified by U.S. Pat. No. 4,075,480. In both of the foregoing techniques the sample is counted twice, once by itself and once while being irradiated by a known or standard radioactive source.
Typically, in each of the above techniques, a standard radioactive source is disposed in a carrier tube or conduit and is shifted back and fourth within the conduit between a shielded or rest location remote from the counting chamber and a counting or operating location adjacent the counting chamber. The radioactive source is usually a source of gamma radiation, such as Cesium-137, incorporated within a suitable container such as a metal ball or pellet which will slide freely within the conduit. When a sample is in position in the counting chamber, the radioactive source is shifted to its operating location to irradiate the sample, and the resulting scintillation (light flashes) emanating from the sample are counted. Thereafter, the source is returned to its shielded location and the scintillations produced solely by the radioactive sample are counted. This two-step counting procedure is repeated for each sample to be analyzed.
While various arrangements for shifting the radioactive source within the conduit have been adopted in commercial liquid scintillation counters, they have not proven satisfactory in all respects. In one approach, a pneumatic pump and a plurality of solenoid-controlled valves are connected to the conduit. In a first operating mode, the pump propels the radioactive source from its shielded location to its operating location. Then, at the conclusion of the counting operation, the valves are selectively actuated to reverse the direction of air flow in the conduit such that the pump propels the source back to its shielded location. The use of solenoid-controlled valves to control the direction of air flow results in a relatively expensive and mechanically complex control system which, in the course of switching operation, introduces undesired noise signals into the scintillation counting system.
A second and simpler approach utilizes only one-way air flow in the conduit to propel the radioactive source to the operating position at the counting chamber. After the sample is counted, the air flow is terminated, and the radioactive standard falls by gravity to the shielded location. Unfortunately, it has been found with this arrangement that the radioactive source often sticks in the operating position. Such sticking is believed due to one or both of: (1) mechanical wedging of the the source against the interior wall of the conduit and (2) static attraction between the source and the conduit wall induced by the air flow. In practice, it has been found that the radioactive source can stick in this manner as often as one time in forty. When this happens, the source remains proximate the counting chamber and continues to irradiate the sample. Since the scintillations produced by the source totally swamp scintillations produced by the sample alone, under such circumstances the second counting step for the two aforementioned quench determining techniques produces totally erroneous counting information. Moreover, the pneumatic system may allow the bead to oscillate or vibrate in the operating location adjacent a sample. In addition, the pneumatic system may not locate the source at exactly the same position for successive samples. An oscillating or othereise inaccurately positioned source can produce errors in the sample counting or calibration procedure. Finally, the radioactive source is subject to shock and vibration during shifting back and forth and particularly when striking stops at the ends of the conduit. This increases the likelihood that radioactive material will escape from the source container.
In another approach for shifting a radioactive source, exemplified in U.S. Pat. No. 3,500,447, the source is supported on the end of a cable, wire, rod, or wand and is inserted in one direction through a passage to a position directly beneath a sample to be counted and then is retracted back along the same passage to a shielded location. This approach has not been widely adopted perhaps because it has close dimensional restrictions in requiring that the source be inserted into a bore of the sample vial elevator to a position directly below the vial.
In addition to the foregoing limitations, the prior approaches exhibit dimensional and operational restrictions of source orientation with respect to the sample to be irradiated generally requiring a relatively high radioactive source strength, typically between 10-40 microcuries, which may require extra shielding and may further necessitate that some users obtain a government license to operate the instrument. The prior approaches are also limited to the positioning of a single radioactive source at a single counting chamber.